Injection Molding
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Dimensions of homopolymer polypropylene injection products continue to change after demolding
Problembeschreibung
Product dimensions continuously change within 24-72 hours after demolding, with obvious difference between final dimensions and dimensions at demolding.
Dimensional instability affects assembly accuracy and product quality, common in crystalline materials.
Ursachenanalyse
(1) High crystallinity of homopolymer polypropylene; continued crystallization after demolding causes post-shrinkage
(2) Demolding temperature too high; product not fully cooled, continues to shrink at room temperature
(3) Insufficient cooling time, crystallization process incomplete
(4) Large ambient temperature changes, product dimensions fluctuate at different temperatures
(5) Internal stress release causes slow dimensional change
Lösung
(1) Extend cooling time, ensure product demolding temperature more than 10°C below heat deflection temperature
(2) Appropriately reduce mold temperature to the lower value within the 40-90°C range, ensure full crystallization
(3) Anneal the product after demolding, hold at 30-50°C below melting point for 2-4h to eliminate internal stress
(4) Control stable storage environment temperature, avoid drastic temperature changes
(5) Establish a dimensional stability inspection system, record dimensional changes at 24h, 48h, 72h after demolding
Wichtigste Erkenntnis
Post-shrinkage is an inherent characteristic of crystalline materials; dimensional stability is ensured through full cooling, annealing treatment, and environmental control.
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